Increasing restrictions on the amount and types of volatiles which can be released in work environments and the desire to reduce energy consumption have prompted the development of radiation curable coating compositions which are essentially free of volatile solvents that must be evaporated during the curing of the composition. These compositions are said to be 100 percent reactive; that is, each component of the composition reacts and becomes incorporated into the cured coating when the composition is exposed to actinic radiation.
The radiation curable coatings which are disclosed in the prior art typically contain a radiation reactive oligomer or resin, a photoinitiator, and, optionally, a radiation reactive diluent and/or a radiation reactive crosslinker.
A variety of materials are disclosed as being useful as the oligomer or resin component of the radiation curable coating compositions. Among these are polyurethane oligomers or resins having terminal acrylyl or methacrylyl groups. These are generally produced by the reaction of one or more organic polyisocyanates with one or more organic polyols, wherein at least a portion of the polyisocyanate or polyol reactant has, in addition to its isocyanate or hydroxyl functionality, acrylyl or methacrylyl groups. The prior art discloses acrylate or methacrylate capped polyurethanes wherein the organic polyol used in their production is a polyester polyol. For example, U.S. Pat. No. 3,700,643 discloses a number of acrylate capped polyurethanes based on polycaprolactone polyols. The prior art also discloses acrylate or methacrylate capped polyurethanes based on polyether polyols (see, e.g. U.S. Pat. Nos. 3,782,961 and 3,955,584).
It is desirable that the acrylate or methacrylate capped polyurethane employed in the radiation curable coating composition have a sufficiently low viscosity that the coating compositions based thereon are easily applied to a substrate using conventional coating techniques without the excessive use of diluents. Although low molecular weight acrylate or methacrylate monomers, such as 2-ethylhexyl acrylate, 2-hydroxyethyl acrylate, etc., can be used as reactive diluents in conjunction with the oligomer or resin, it is desirable to use as little of these monomers as possible, since these monomers are somewhat toxic and special care must be taken to avoid skin contact with them.
The known acrylate or methacrylate capped polyurethanes based on polyether polyols exhibit desirable low viscosity, however, the radiation cured coatings produced from them do not exhibit the high level of tensile strength, light stability or other performance properties necessary in many end use applications. The known acrylate or methacrylate capped polyurethanes based on polyester polyols impart excellent physical properties to radiation cured coatings, however, they have relatively high viscosities so that undesirably high amounts of the aforementioned diluents must be used in order to achieve the desired application viscosity.
The radiation curable coatings art is seeking a radiation reactive oligomer or resin which has relatively low viscosity, preferably below about 50,000 cps. (23.degree. C.), which, at the same time, imparts good physical properties to a cured coating.